Salt-resistant alpha-helical cationic antimicrobial peptides.

Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.

Abstract

Analogues based on the insect cecropin-bee melittin hybrid peptide (CEME) were studied and analyzed for activity and salt resistance. The new variants were designed to have an increase in amphipathic alpha-helical content (CP29 and CP26) and in overall positive charge (CP26). The alpha-helicity of these peptides was demonstrated by circular dichroism spectroscopy in the presence of liposomes. CP29 was shown to have activity against gram-negative bacteria that was similar to or better than those of the parent peptides, and CP26 had similar activity. CP29 had cytoplasmic membrane permeabilization activity, as assessed by the unmasking of cytoplasmic beta-galactosidase, similar to that of CEME and its more positively charged derivative named CEMA, whereas CP26 was substantially less effective. The activity of the peptides was not greatly attenuated by an uncoupler of membrane potential, carbonyl cyanide-m-chlorophenylhydrazone. The tryptophan residue in position 2 was shown to be necessary for interaction with cell membranes, as demonstrated by a complete lack of activity in the peptide CP208. Peptides CP29, CEME, and CEMA were resistant to antagonism by 0.1 to 0.3 M NaCl; however, CP26 was resistant to antagonism only by up to 160 mM NaCl. The peptides were generally more antagonized by 3 and 5 mM Mg2+ and by the polyanion alginate. It appeared that the positively charged C terminus in CP26 altered its ability to permeabilize the cytoplasmic membrane of Escherichia coli, although CP26 maintained its ability to kill gram-negative bacteria. These peptides are potential candidates for future therapeutic drugs.

Survival of logarithmic-phase (A) and stationary-phase (B) E. coli UB1005 in MH broth after addition of fourfold the MIC of peptide, gentamicin, or ceftazidime. This corresponds to 2 μg of CP26, CP29, and ceftazadime per ml, 4 μg of CEME and CEMA per ml, and 0.5 μg of gentamicin per ml. Actual initial concentrations of bacteria ranged from 0.5 × 108 to 2.5 × 108/ml but were corrected to an initial concentration of 1 × 108/ml for clarity. A typical experiment out of three trials is shown. Symbols: ▵, no peptide; ▴, CP26; ▾, CP29; ●, CEME; ○, CEMA; ■, ceftazidime; □, gentamicin. Results for CP26 in panel A were almost superimposable with the ceftazidime results and were thus omitted for clarity. No evidence of bacterial aggregation was observed when viewed under a light microscope or in light-scattering experiments.